Great Britain is defined as a political entity by the Act of Union 1707. The later Act of Union with Ireland created a single United Kingdom but this was between Great Britain and Ireland (later Northern Ireland) and from this time the two Kingdoms became a single political entity so Scotland would need to no longer be part of Great Britain boths Acts are to remain unchanged.
Well, yes, you may have a point about Great Britain being a political entity in the terms of the Act.
That said, I still believe that the 1707 Act would have to be repealed in it's entirety for the 1801 act to be automatically voided. Without this, a Scottish Independence Act would just have the effect of modifying the 1707 Act so that as far as the 1801 Act is concerned, England/Wales would be the successor to "Great Britain".